1. Field of the Invention
This invention relates to heat dissipation of a semiconductor device or electronic module, and in particular to a method and means for accomplishing such dissipation and reducing stresses caused by thermal gradients by employing an electronic module cap and heat exchanger as a single structure, and a semi-molten material between the IC chip and its associated heat exchanger to melt and solidify in the IC chip operational temperature range in order to release and absorb heat energy and create a low thermal resistance path to the heat exchanger.
2. Description of Related Art
Technological advancements in semiconductor device design and fabrication have significantly increased integrated circuit chip power requirements, and these requirements continue to climb. In order to accommodate these enhanced power requirements, it has become necessary to find more efficient ways to dissipate heat through the electronic packages and their associated heat exchangers.
Current electronic module designs do not efficiently dissipate heat energy at rates beyond power levels of 25 to 50 Watts. Beyond these levels expensive methods of water cooling are necessary to minimize the external thermal resistance and maximize the thermal gradient, thus enhancing heat flow. The power dissipation is inherently limited by both internal and external thermal resistance to heat flow. Thermal pastes currently in use as low thermal resistant conductors between an IC chip and its heat exchanger are adequate to satisfy the thermal dissipation requirements at the present state of the art, however, they are inadequate for the much higher power allotments being contemplated for future designs. Although there are many materials, especially metals, that have excellent thermal conductivity satisfying the thermal dissipation requirements, most of the metals in the solid state induce intolerable stresses in the multi-chip modules during thermal cycling. The conductive material used for this heat transfer is typically placed between the IC chip and the heat exchanger, and consequently may be exposed to considerably different thermal expansion coefficients (TECs) and/or different temperatures due to thermal gradients. The result is IC chip cracking, unacceptable C-4 solder deformation, and excessive fatigue damage leading to early failure.
There is also the associated problem of operational thermal cycling or mini-cycles that are common among devices employing power saving schemes, such as CMOS technology parts. The alternating power demands by the IC chip exacerbate the power dissipation requirements and enhance device fatigue. Thus, in order to accommodate a new generation of devices and establish a more efficient thermal transfer from the IC chip to its heat exchanger, the effects of mini-cycles must also be attenuated.
To prevent an IC chip from exceeding its prescribed operating temperature, it is necessary to maximize the heat flow across the interface to the chip's heat exchanger such that heat may be effectively conducted away from the chip. A key concern with high heat generating chips is the achievement of very low and stable chip thermal contact resistance. This requires the use of a thermal dissipation technique that embodies thermal and mechanical properties such that uniform contact loading and low interface thermal resistance are continuously maintained. The prior art has considered a number of techniques towards this objective. Most can be classified as one of either a solid metallurgical bond, a microstructural or structural contact, a demountable liquid or grease interface, or a combination of these types.
In U.S. Pat. No. 5,170,930 issued to Dolbear, et al. on Dec. 15, 1992, entitled "LIQUID METAL PASTE FOR THERMAL AND ELECTRICAL CONNECTIONS", a thermally and electrically conductive paste is used for cooling electronics. The conductive and compliant paste are connected between the IC chip surface and the heat exchanger surface. It consists of an equilibrium mixture of liquid metal with particulate solid constituents, such as particles or fibers. The materials added to the liquid metal as well as their compositions are selected such that a paste is formed instead of a permanent solid. Various types of liquid metals are suggested, including gallium, indium, mercury, cadmium and bismuth. Also, numerous powder additives are employed such as alumina, aluminum, aluminum nitride, chromium, gold, lead, and silicon, to name a few. The application of the paste is for thermal conduction purposes. As such, the paste does not undergo a change of state, i.e., melt and re-solidify under the IC chip operational conditions. The mixture does not provide the melting and solidifying properties in the IC chip operational temperature range for which the material must have the properties of releasing or absorbing sufficient heat to counteract the effects of mini-cycles.
In U.S. Pat. No. 5,325,265 issued to Turlik, et al. on Jun. 28, 1994, entitled "HIGH PERFORMANCE INTEGRATED CIRCUIT CHIP PACKAGE", cushions formed of thermally conductive low melting point material are placed between the heat sink and the IC chip for the transfer of heat from the chip to the heat exchanger. The cushion is sufficiently thick to be able to absorb movement (dimensional variations) between the IC chip and the heat exchanger during thermal cycling, yet sufficiently thin to act as a thermal conductor between the chip and the heat exchanger. In forming this package, the stress absorbing cushions are designed to have the lowest melting point or deformation temperature of all components used. Once brought together, the entire assembly is heated above the melting temperature of the cushion. The cushions melt and reflow to form conformal cushions between the heat exchanger and the IC chip accommodating the separation irregularities. The preferred material for the cushion is indium or an alloy of indium. Once again, this application is for thermal conduction purposes. The solder is not designed to melt and re-solidify under the IC chip operational temperature range which would absorb and reject heat. Thus, this art does not have the capacity for heat generation and subtraction during the rise and fall of temperature due to mini-cycles. As such, the mini-cycle thermal excursions are neither suppressed or attenuated.
Lastly, in U.S. Pat. No. 4,607,277 issued to Hassan et al. on Aug. 19, 1986, entitled "SEMICONDUCTOR ASSEMBLY EMPLOYING NONEUTECTIC ALLOY FOR HEAT DISSIPATION", an alloy was employed to form a low thermal resistance bridging interface between the surface of an IC chip and the surface of a heat exchanger. The alloy has a solidus-liquidus temperature range such that the solidus is slightly below the maximum operating temperature of the chip, and thus has the capability to reestablish and maintain the interface at a low thermal resistance if stressed during circuit operation. The alloy, however, which is preferably a composition of bismuth, lead, tin and indium, initially only makes contact with the surface of the chip at spaced points between voids. During IC package assembly, the temperature of the device is raised to the melting range of the alloy. The bismuth alloy then melts and conforms under viscous flow conditions to fill the void spaces between the chip and the heat exchanger. Disruptions due to dimensional variations, differential thermal expansions, and impact load effects will increase the thermal resistance due to the reoccurrence of voids. Thus, the temperature will rise into the melting range of the alloy, thereby recovering the low thermal resistance. In effect, a self-healing process of the alloy melting to conform to the chip surface occurs so that the contact thermal resistance is maintained at a low level. Similar to the above referenced inventions, this prior art also has the objective of dissipating heat more efficiently by minimizing interfacial thermal resistance. Here, it is done with the use of molten solder. The solder, however, remains a distance away from the chip and would not be able to significantly attenuate the temperature cycling due to mini-cycles even if the solder were suitable and had the requisite thermodynamic properties to meet the requirements of power dissipation in the proper temperature range. Thus, the patent is strictly one for improving heat conduction through the improvement of the thermal resistance path.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus for the removal of heat from an integrated circuit chip to ambient air that attenuates the effects of thermal mini-cycles associated with IC chip operation.
It is another object of the present invention to provide a method and means for removing heat from an integrated circuit chip without incurring thermally induced shear stress on the chip or the accompanying substrate.
A further object of the invention is to provide a low thermal resistance path between the IC chip and its associated heat exchanger.
It is yet another object of the present invention to provide an IC chip package in which thermal mismatches between the chip, its associated heat exchanger, and an interfacing thermal conducting material are minimized.
Yet another object of the present invention is to provide a more efficient way to dissipate heat from an integrated circuit chip through its associated heat exchanger and the joint bond there between.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.